High temperature pressure transducer employing a metal diaphragm

ABSTRACT

A pressure transducer includes a diaphragm, having an active region, and capable of deflecting when a force is applied to the diaphragm; and a sensor array disposed on a single substrate, the substrate secured to the diaphragm, the sensor array having a first outer sensor near an edge of the diaphragm at a first location and on the active region, a second outer sensor near an edge of the diaphragm at a second location and on the active region, and at least one center sensor substantially overlying a center of the diaphragm, the sensors connected in a bridge array to provide an output voltage proportional to the force applied to the diaphragm. The sensors are dielectrically isolated from the substrate.

FIELD OF THE INVENTION

This invention relates to pressure transducers and more particularly toa high temperature pressure transducer.

BACKGROUND OF THE INVENTION

It has been a desire to provide pressure transducers which can operateat high temperatures and which basically are simple to construct. Theprior art is replete with patents and devices which can operate atrelatively high temperatures. Such devices include silicon sensors whichoperate for example at temperatures of 600° C. or higher. Other patentswhich are assigned to the assignee herein, namely Kulite SemiconductorProducts, Inc., depict silicon carbide transducers which are capable ofextremely high temperature operation. The prior art is replete withpatents which utilize metal as deflecting diaphragms. Onto these metaldiaphragms are affixed piezoresistive or other type of sensors. Thediaphragms being made of metal are capable of operating at hightemperatures. Sensors affixed to the diaphragm can include wire straingauges or other types of semiconductor strain gauges. Such gauges haveto be placed onto the diaphragm in specific positions and eachindividually affixed to the diaphragm at specific positions.Accordingly, the placement of the sensors on the diaphragm can be a timeconsuming task.

In the prior art, there were two principal methods of making miniaturehigh temperature pressure transducers. In the first method, a thinmetallized isolation diaphragm was mounted in front of the sensor andthe pressure was transmitted to the sensor by a small volume of oil. Aspressure was applied to the isolation diaphragm, pressure wastransmitted to the sensor by the oil, which is a non-compressible fluid.As long as the oil retains its property as a non-compressible fluid,such a pressure transducer operates properly. However, as thetemperature increases, the vapor pressure of the oil increases to apoint where the oil no longer transmits the pressure to the sensor,setting an upper temperature limit on the operation of the transducer.In the second method, as individual gauges are affixed to the diaphragm,using either a high temperature cement or glass. In this method the onlydielectric isolation between the gauge and the metal diaphragm is thecement itself. Any variations in the thickness of the cement can causeelectrical breakdowns at relatively low temperatures, causing thetransducer to fail. Furthermore, if the individual gauges are made fromsilicon, in order to obtain sufficient resistance, the gauge must berather long and exhibit rather high resistivity. Thus it becomesdifficult to place the individual gauge in a resistor in a region ofhigh stress. As the temperature is increased, the relatively highresistivity material used for the sensor changes its value at anon-linear rate as the temperature is increased to a higher value,making thermal compensation very difficult. Moreover, each individualgauge must be separately applied to the diaphragm, or alternatively, ifthe various sensors are interconnected prior to application to thediaphragm, the structure is complex, fragile, and difficult to handle,so that the resulting transducers are of dubious quality.

SUMMARY OF THE INVENTION

A pressure transducer in accordance with an embodiment of the inventionincludes a diaphragm, having an active region, and capable of deflectingwhen a force is applied to the diaphragm; and a sensor array disposed ona single substrate, the substrate secured to the diaphragm, the sensorarray having a first outer sensor near an edge of the diaphragm at afirst location and on the active region, a second outer sensor near anedge of the diaphragm at a second location and on the active region, andat least one center sensor substantially overlying a center of thediaphragm, the sensors connected in a bridge array to provide an outputvoltage proportional to the force applied to the diaphragm.

A sensor array in accordance with an embodiment of the inventionincludes an elongated substrate having a top and a bottom surface andopposing ends, and having a sensor array located on the top surface, afirst semiconductor sensor on the top surface close to one of theopposing ends and a second semiconductor sensor on the top surface closeto the other of the opposing ends, and two semiconductor sensorspositioned substantially at a center of the substrate, the sensorsconnected together by contact areas located on the substrate to form abridge configuration.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross-sectional view of a pressure transducer taken throughline 1-1 of FIG. 2 and in accordance with an embodiment of thisinvention.

FIG. 2 is a top view of a pressure transducer in accordance with anembodiment of this invention.

FIG. 3 is a top plan view of a sensor array of the transducer of FIG. 2,taken along line 3-3 of FIG. 2.

FIG. 3A is an enlarged view of the portion of FIG. 3 as indicated inFIG. 3.

FIG. 3B is a schematic showing a Wheatstone bridge array formed by thesensor array of FIG. 3.

FIG. 4 is a view of a typical masked assembly utilized to produce thearray of FIG. 3.

FIG. 5 shows multiple arrays similar to those of FIG. 3 formed on asingle semiconductor wafer.

FIG. 6 shows the array of sensor arrays of FIG. 5 separated intoindividual arrays for use on separate metal diaphragms.

FIG. 7 shows a cross-sectional view of the sensor array of FIG. 3secured to a metal diaphragm by a high temperature glass frit.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2 there is shown a pressure transducer inaccordance with an embodiment of the present invention. FIG. 2 depicts apressure transducer 10, which has a housing 25 and which has a diaphragm12. FIG. 1 depicts a cross-sectional view taken through line 1-1 of FIG.2. Diaphragm 12 may be of metal and may be of uniform thickness. Theparticular thickness, radius and material may be selected depending onthe particular application. Diaphragm 12 may be supported exclusively ata circumference thereof, and may be supported around the entirecircumference thereof. In the absence of an applied force, diaphragm 12is planar or substantially planar. In the presence of an applied force,diaphragm 12 is capable of deflection. An active area of diaphragm 12deflects upon application of a force. Diaphragm 12 may be a continuousend wall of a closed cylindrical member 15 having an inner diameterapproximately one-half that of its outer diameter. If closed cylindricalmember 15 is circular, diaphragm 12 may be circular. FIG. 2 also depictsin dashed lines sensor array 20. Diaphragm 12 has a top surface externalto transducer 10, and a bottom surface interior to transducer 10. Sensorarray 20 is affixed to the bottom surface of the active area ofdiaphragm 12.

As one can see from FIG. 1, sensor array 20 is completely isolated fromthe environment external to transducer 10 by closed cylindrical member15. Leads from sensor array 20 are directed to terminal pins, 17 and 18,which pins connect with connecting terminal port 30, which is in turnconnected to output connector 14.

Referring now to FIG. 3, sensor array 20 contains four sensors 25, 27,41, 42, which are interconnected to form a bridge circuit, such as aWheatstone bridge. Sensors 25, 27, 41, 42 may be piezoresistive devices.As explained below in greater detail, sensor array 20 is so secured tothe bottom surface of diaphragm 12 that deflection of diaphragm 12causes sensors 25, 27, 41, 42 to change resistance. Thus, resistancechanges upon application of an applied force F as shown in FIG. 1 to thetop surface of diaphragm 12. Sensor array 20 is dielectrically isolatedfrom diaphragm 12. Sensor array 20 is defined on a substrate. Thesubstrate may be a wafer, such as a single-crystal silicon wafer.

It is well known that when a circular deflecting diaphragm having aclamped edge is exposed to a pressure normal to its surface, the areasnear the clamped edge experience compressive stresses while in thecentral region the area experiences tensile stresses. These stresses aregiven by the following formula.

$\begin{matrix}{\sigma = {3P\frac{\left\lbrack {{a^{2}\left( {1 + v} \right)} - {v^{2}\left( {3 + v} \right)}} \right\rbrack}{8t_{D}^{2}}}} & (1)\end{matrix}$

Where P is the applied pressure, a is the radius of the diaphragm, ν isPoisson's ratio, and t_(D) is the thickness of the diaphragm.

As can be seen from the dashed line depiction in FIG. 2, sensor array 20has an elongated form, in which sensor array 20 extends across diaphragm12 substantially from one edge to an opposing edge, while sensor array20 is relatively narrow. Sensor array 20 may be in the form generally ofa narrow rectangle. The length of sensor array 20 is less than thediameter of an active region of diaphragm 12. Sensor array 20 may be asilicon-on-insulator (SOI) structure. Each sensor may be a P+piezoresistor element, each separated by an oxide layer from theunderlying silicon substrate. The substrate may be affixed to diaphragm12 by a high temperature glass frit. High temperature glass frits arewell known in the art and many patents assigned to the assignee of thepresent application, Kulite Semiconductor Products, Inc., disclose suchfrits as well as the composition of the same.

Referring again to FIG. 3, first outer sensor 25 is at, or close to, afirst end of sensor array 20. Second outer sensor 27 is at, or close to,a second end of sensor array 20. First and second ends are opposingends. First and second outer sensors 25, 27, are each positioned near anedge of diaphragm 12, and are on the active region of diaphragm 12.First outer sensor 25 is near an edge of diaphragm 12, at a firstlocation, and second outer sensor 27 is also near an edge of diaphragm12, at a second location. First outer sensor 25 and second outer sensor27 are thus both near an edge of diaphragm 12, but at differentlocations, and may be positioned at locations on opposite edges ofdiaphragm 12. First center sensor 41 and second center sensor 42 arelocated at the center of sensor array 20, and overlying or near a centerof diaphragm 12. Center sensors 41 and 42 are interconnected on thearray by, for example, suitable lead areas which are deposited on thearray.

Continuing to refer to FIG. 3, first outer sensor 25 is associated withcontact 22 which connects one terminal of first outer sensor 25 to oneterminal of first center sensor 41. The other terminal of first outersensor 25 is connected by a contact 21 to one terminal of second centersensor 42. In a similar manner, one terminal of first center sensor 41is connected to contact 28. The other terminal of second center sensor42 is connected to contact 23 which connects to one terminal of secondouter sensor 27. The other terminal of second outer sensor 27 isconnected to contact 24. The schematic or equivalent diagram of sensorarray 20 depicted in FIG. 3 is shown in FIG. 3B. As is evident from FIG.3B, sensors 25, 27, 41, and 42, which, as noted, are piezoresistors,define a bridge circuit, and more particularly a Wheatstone bridge.Contacts 24, 28 are the output leads of the circuit defined by sensorarray 20. Contacts 24, 28, are connected to cable 14 of FIG. 1. Theoutputs from contacts 24 and 28 may be connected to various devices andcircuits for compensating for such factors as variation in temperature.Such devices may include, by way of example, compensating resistors. Asone can also see from FIG. 3B, a biasing potential may be applied tocontact 22 indicated as +IN; a reference potential may be applied tocontact 23 which is indicated as −IN. One output of sensor array 20 istaken from terminal 21, indicated as −OUT, while the other output istaken between terminals 28 and 24, indicated as +OUT.

Shown in FIG. 3A, is an enlarged view of an exemplary embodiment ofsecond end sensor 27. Second end sensor 27 includes a series ofinterfolded or interconnected lines in a serpentine pattern. Such aseries of interfolded or interconnected lines may be advantageous inproviding a resistor of relatively high value in a relatively smallarea. FIG. 3 includes exemplary dimensions of such sensors and of asensor array 20. By way of example, a piezoresistor employed as a sensormay have dimensions of about 7.6 mils by 2.76 mils. As one can see fromFIG. 3 the length of the array from the front of piezoresistor 25 to theback of piezoresistor 27 is typically 91 mils.

The sensor array may be fabricated on a wafer of silicon. The wafer ofsilicon has deposited thereon a layer of silicon dioxide, and sensors25, 27, 41 and 42 are deposited on the layer of silicon dioxide bysuitable techniques. The sensor array itself may be termed a SOIstructure with four P+ resistor elements separated by an oxide layerfrom the underneath silicon material. Thus, each sensor isdielectrically isolated from the silicon substrate.

FIG. 7 is a sectional view through diaphragm 12 and sensor array 20.Secured to metal diaphragm 12 is sensor array 20. Sensor array 20 isprovided on substrate 101, which may be a wafer of single-crystalsilicon, for example. Substrate 101 is secured to diaphragm 12 by a hightemperature glass frit 102. Silicon substrate 101 has a layer 100 ofsilicon dioxide formed thereon. Upon layer 100 of silicon dioxide arefirst and second outer sensors 25 and 27 and center sensors generallydesignated 26, and contacts 21, 23.

Methods for fabrication of the devices described herein are well knownto those of skill in the art. By way of example, the assignee of thepresent application, Kulite Semiconductor Corp., has many patents whichteach techniques for fabrication of SOI devices which techniques areamong those which may be employed to fabricate sensor array 20. See, forexample, U.S. Pat. No. 5,973,590 entitled “Ultra Thin Surface MountSensor Structures and Methods of Fabrication” issued on Apr. 3, 2001 andassigned to Kulite Semicondcutor Products, Inc. the assignee herein.

Sensor array 20 is defined on substrate 101. The shape and dimensions ofsubstrate 101 are adapted to permit outer sensors 25, 27 to be formedclose within the active region of diaphragm 12 and close to thecircumference of diaphragm 12, while permitting center sensors 41, 42,to be formed close to the center of diaphragm 12. In the illustratedembodiment, this is accomplished by providing substrate 101 to have agenerally rectangular form having a first width, and having an endwidth, narrower than the first width, at each end to permit outersensors 25, 27 to be closer to the curved circumference of diaphragm 12.This also enhances the amount of stress induced in outer sensors 25 and27. In a central region, the width of substrate 101 may be a centerwidth less than the first width, to enhance the amount of stress inducedin center sensors 41, 42.

The dimensions described below and illustrated in FIG. 3 provide anon-limiting example of dimensions for a sensor array for use with acircular diaphragm 12 having a diameter of 100 mils. Substrate 101 maybe approximately 95 mils long, by approximately 20 mils wide. The sensorarray 20 may be 91 mils long by 13.5 mils wide. The areas of reducedwidth containing outer sensors 25 and 27 may be 12-13 mils wide. Thearea of reduced width at the center containing center sensors 41, 42 maybe about 10 mils wide for a length of about 10 mils.

According to the above-noted stress equation, stress at any given pointon diaphragm 12 is dependent on the distance of the point from thecenter of the diaphragm, and decreases significantly as the distancefrom the center of the diaphragm decreases. In fact the stress actuallychanges in sign at about ⅔ of the radius. Thus, as the size of thesensors, particularly along the axis of the diaphragm, is keptrelatively small, averaging of the detected stress at any given sensoris reduced. By way of example, in a 100 mil diameter diaphragm, theresistor length may be no more than about 10 mils.

FIG. 3A shows in detail an exemplary structure of outer sensor 27, andterminals coupled to contacts 23 and 24. The other sensors may have thesame structure. The line widths of sensor 27 may be extremely small,such as about 0.17 mils. The spacing between each line may be about 0.2mils. In the illustrated example, since the separation of the lines ison the order of 1 mil, with 5 back and forth lines, the resistance maybe about 2000 ohms or in excess of about 2000 ohms, in a width of lessthan about 5 mils. The width of the resistor as depicted in FIG. 3A isabout 3 mils.

One obtains a high resistance in a small area, such as a resistance inexcess of 2000 ohms, in an area of about 5 mils by about 7.6 ohms. Sucha high resistance is advantageous in that there is relatively littleself heating, which tends to provide a more stable transducer.

A variety of methods for fabricating sensor array 20, which may be adielectrically isolated SOI sensor bridge array as depicted in FIG. 3,are known in the art. One exemplary technique commences with twosubstrates. The two substrates may be wafers, such as wafers of singlecrystal silicon. A first wafer may be designated as a pattern wafer andmay be selected to optimize the piezoresistive performancecharacteristics of each of the sensors as 25, 27, 41 and 42. The secondwafer also fabricated from silicon is designated as a substrate waferand acts as a base. The thickness of the substrate wafer is reducedduring the process, as described in greater detail below.

An oxide layer, such as oxide layer 100 of FIG. 7, may be thermallygrown on the surface of the substrate wafer. Separately, the patternwafer may be patterned with the piezoresistive pattern of the sensors.The piezoresistive patterns may be diffused to the highest concentrationlevel (solid solubility) in order to achieve the most stable, long termelectrical performance characteristics of the sensing network.

Once the pattern and the substrate wafers are appropriately processed,the two wafers may be fusion bonded together using a diffusion enhancedbonding technique. Such a technique is shown in U.S. Pat. No. 5,286,671designated as “Fusion Bonding Technique for Use in FabricatingSemiconductor Devices” issued on Feb. 15, 1994, to A. D. Kurtz et al andassigned to the assignee herein.

The resulting molecular bond between the two wafers is as strong as thesilicon itself. Since both the lines of the sensors and the base are ofthe same silicon material, there is no thermal mismatch between the two,thus resulting in a very stable and accurate performance characteristicwith temperature. The presence of dielectric isolation enables thesensor bridge array to function at very high temperatures without anycurrent leakage effects which are typically associated with ordinary pnjunction type devices.

Referring to FIG. 4 there is shown an exemplary masked wafer for forminga sensor array such as array depicted in FIG. 3. As seen in FIG. 4,there is a surrounding layer 100 which may be a P+ material; layer 100may be used to separate various arrays which are formed on a commonwafer as will be explained. FIG. 4 utilizes the same reference numeralsas depicted in FIG. 3 to designate the same parts. It is understood thatmultiple masks may be employed in order to fabricate the entirestructure but such techniques are well known to those skilled in theart.

FIG. 5 shows a pattern 500 for multiple sensor arrays all fabricated ona single wafer. The wafer can be sliced to separate the sensor arrays,after measurement of their electrical characteristics, for fabricationof multiple transducers. and which sensor arrays can be separated aftertheir electrical characteristics are measured. In this manner, one canproduce numerous sensor arrays as the array of FIG. 3 on one commonwafer and select all arrays which are within desired specifications. Thefabrication of multiple semiconductor devices or multiple devices on asingle wafer is well known. Thus FIG. 5 shows for example a portion ofthe semiconductor wafer which contains 12 sensor arrays as the array ofFIG. 5.

FIG. 6 shows such sensor arrays 505 after a step of separation.

As depicted in FIG. 3, an entire Wheatstone bridge array is provided, ona substrate of silicon having an oxide layer, with the piezoresistors ofthe Wheatstone bridge array deposited thereon. The oxide layer providesisolation between the silicon and the sensing devices. The use ofdielectric isolation enables the sensor bridge to function at very hightemperatures without any current leakage effects, in contrast to pnjunction type devices.

A sensor array in accordance with the invention has many advantages overprior art devices, including the exemplary advantages described herein.In a sensor array in accordance with an embodiment of the invention, thegeometry of the individual sensor elements may be defined byphotolithographic techniques, so that the widths and thicknesses of thesensors can be on the order of fractions of a mil; as a result, verycompact sensors can be formed. As each piezoresistive sensor may be madeof a series of P+ regions of width of about 0.1 mils, a thickness ofabout 0.05 mils and lengths of about 7.5 mils, each sensor may have arelatively high resistivity. For example, each back and forth leg of asensor, with P+ doping densities of above 10²⁰ boron atoms/cm³, may havea resistance in excess of 350 ohms. Since the separation of the legs maybe on the order of 1 mil, and there may be, for example, 5 back andforth legs in a single sensor, such a sensor may provide a resistance inexcess 2000 ohms in a width of less than 5 mils. Such a high resistancereduces self heating and thus provides a more stable transducer.

While the foregoing invention has been described with reference to theabove embodiments, various modifications and changes can be made withoutdeparting from the spirit of the invention. Accordingly, all suchmodifications and changes are considered to be within the scope of theappended claims.

1. A pressure transducer, comprising: a diaphragm, having an activeregion, and capable of deflecting when a force is applied to saiddiaphragm, a sensor array disposed on a single substrate, said substratesecured to said diaphragm, said sensor array having a first outer sensornear an edge of said diaphragm at a first location and on said activeregion, a second outer sensor near an edge of said diaphragm at a secondlocation and on said active region, and at least one center sensorsubstantially overlying a center of said diaphragm, said sensorsconnected in a bridge array to provide an output voltage proportional tosaid force applied to said diaphragm.
 2. The pressure transduceraccording to claim 1, wherein said substrate is a wafer of silicon, andeach of said sensors is a P+ piezoresistor.
 3. The pressure transduceraccording to claim 2, wherein each of said sensors is positioned on anoxide layer, said oxide layer being on said silicon wafer.
 4. Thepressure transducer according to claim 3, further comprising a grassfrit intermediate said wafer and said diaphragm.
 5. The pressuretransducer according to claim 1, further comprising a second centersensor substantially overlying the center of said diaphragm, whereineach of said sensors is a piezoresistive sensor, said sensors beinginterconnected to define a Wheatstone bridge.
 6. The pressure transduceraccording to claim 1, wherein said substrate has opposing ends, saidfirst outer sensor being at one of said opposing ends and said secondouter sensor being at the other of said opposing ends, said substratehaving a main portion width and being relatively narrow at said opposingends.
 7. The pressure transducer according to claim 6, wherein saidsubstrate has a center width less than said first width extending alongthe length of said center sensors.
 8. The pressure transducer accordingto claim 1, wherein said diaphragm comprises an end wall of a closedcylinder.
 9. The pressure transducer according to claim 8, wherein saiddiaphragm is of metal.
 10. A sensor array, comprising: an elongatedsubstrate having a top and a bottom surface and opposing ends, andhaving a sensor array located on said top surface, a first semiconductorsensor on said top surface close to one of said opposing ends and asecond semiconductor sensor on said top surface close to the other ofsaid opposing ends, and two semiconductor sensors positionedsubstantially at a center of said substrate, said sensors connectedtogether by contact areas located on said substrate to form a bridgeconfiguration.
 11. The sensor array according to claim 10, wherein saidsubstrate is silicon, and said top surface having a layer of silicondioxide thereon.
 12. The sensor array according to claim 11, whereinsaid sensors are silicon piezoresistors.
 13. The sensor array accordingto, claim 12, wherein said sensors are P+ silicon piezoresistors. 14.The sensor array according to claim 10, wherein said substrate isgenerally rectangular having a main portion width and being relativelynarrow at said opposing ends.
 15. The sensor array according to claim14, wherein said substrate has a center width less than said mainportion width at the center of said substrate.
 16. The sensor arrayaccording to claim 10, further comprising: a diaphragm having a top andbottom surface and of a diameter slightly greater than a length of saidsubstrate, said array secured to said bottom surface of said diaphragm.17. The sensor array according to claim 16, wherein said diaphragm is aportion of the top surface of a cylindrical metal member.
 18. The sensorarray according to claim 16, wherein said substrate is bonded to saiddiaphragm by a high temperature glass frit.
 19. The sensor arrayaccording to claim 10, wherein said bridge configuration is a Wheatstonebridge.
 20. The sensor array according to claim 10, wherein said sensorsare piezoresistors having a line in a serpentine pattern.